PSN-L Email List Message

Subject: Re: Seismic intrusion detectors part 2
From: ChrisAtUpw@.......
Date: Wed, 9 Apr 2003 15:54:22 EDT


In a message dated 09/04/03, Mike@........................ writes:

> The sensor consists of (2) geophones.  They are places aprox. 30-40 feet 
> apart on a known trail that illegals use. 
>  
> The prototypes we are using have (2) LT1677 opamp circuits that amplify the 
> geophone outputs and feed the signal into a basic stamp 1.  The level of 
> signal isn't converted to digital, the signal when large enough (about 1.67 
> volts) will drive the input pin high and thus we have a logic switch.
> During initial powerup we callibrate the instrument by "walking the line" 
> or walking by both geophones.  By doing this and counting the number of 
> footfalls on an average man we can the have a rudimentary counting of the 
> number of people walking by.  By dividing the number of detected footfall 
> by the number of detected footfalls in the calibration we have the number 
> of intruders.  It all works but the noisy circuit is killing me.
>  
> This system works but our circuit is noisy and when I make the software 
> just send hits on either geophone as they come in, sometimes, at random,  
> one or both of the "ports" will show hits for minutes on end.  If I touch 
> the circuit board with my finger i can kill the noise until some kind of 
> 

Dear Michael King,

       The LT1677 is one of the best opamps for your job. From your 
description of the way the noise stops when you touch the board, I strongly 
suspect that you have faulty boards. It could be a faulty connection, faulty 
soldering, faulty components, poor design, something unstable in the circuit 
allowing oscillations, radio interference..... You need to get someone to 
find out, using an oscilloscope, just what is happening and then track down 
the why. What I am certain about, is that it should not be happening... and 
it does not happen in my geophones. One test that I have applied is to use a 
small battery powered audio amplifier with headphones to listen to the output 
of the circuit.  

       Regards,

       Chris Chapman
In a message dated 09/04/=
03, Mike@........................ writes:


The sensor consists of (2)=20= geophones.  They are places aprox. 30-40 feet apart on a known trail th= at illegals use.=20
=20
The prototypes we are using have (2) LT1677 opamp circuits that amplify=20= the geophone outputs and feed the signal into a basic stamp 1.  The lev= el of signal isn't converted to digital, the signal when large enough (about= 1.67 volts) will drive the input pin high and thus we have a logic switch.
During initial powerup we callibrate the instrument by "walking the line= " or walking by both geophones.  By doing this and counting the number=20= of footfalls on an average man we can the have a rudimentary counting of the= number of people walking by.  By dividing the number of detected footf= all by the number of detected footfalls in the calibration we have the numbe= r of intruders.  It all works but the noisy circuit is killing me.
=20
This system works but our circuit is noisy and when I make the software=20= just send hits on either geophone as they come in, sometimes, at random, &nb= sp;one or both of the "ports" will show hits for minutes on end.  If I=20= touch the circuit board with my finger i can kill the noise until some kind=20= of seismic activity start the noise again.


Dear Michael King,

      The LT1677 is one of the best opamp= s for your job. From your description of the way the noise stops when you to= uch the board, I strongly suspect that you have faulty boards. It could be a= faulty connection, faulty soldering, faulty components, poor design, someth= ing unstable in the circuit allowing oscillations, radio interference..... Y= ou need to get someone to find out, using an oscilloscope, just what is happ= ening and then track down the why. What I am certain about, is that it shoul= d not be happening... and it does not happen in my geophones. One test that=20= I have applied is to use a small battery powered audio amplifier with headph= ones to listen to the output of the circuit.  

      Regards,

      Chris Chapman

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